It's a rarely used combination as there's no real point in it. You use drivers of different sizes to get the different characteristics that each offers, which simply put is bigger for lower frequencies, smaller for higher frequencies and wider dispersion of same. The size differential should be at least a 4:1 ratio, though that tends to be more like 2:1 in bass applications. The responses of 12's and 15's have so much overlap there's little to recommend the combination. A fifteen and an eight is a much better choice.

i use an Avatar CB112H and CB115H { when i'm not using my Bergantino HT310 } ...
they sound great as a combination .
cabinets are tuned to 51 Hz. , but sound really deep and mellow .

the 15" has plenty of bottom and a real nice mid honk { tweeter
is run at about 3 o'clock } .
the 12" is mostly used as a side shot for my drummer , but he
says it cuts thru better than any monitor he's tried .
we play mostly blues and funk ...

both cabinets are rated at 600 watts @ 8 ohm , and i push them
with a QSC RMX2450 power amp .

I use a huge Trace 15", which is very dark sounding by itself, together with a home build 12" which happens to be very bright for a 12", with good midrange.

For just solid grunting (old style rock), I can do with the Trace alone. Small jazz gigs are cool with the 12". Together they rock tha house. For me, the 15" gives great foundation, the 12" makes it cut through.

Not only am I using a tiny 1-15 and 1-12, they're rear ported eek: ) AND they work great outside! Instead of trying to judge them based on specs and coming up with an equation on their likelyhood of sonic success I went the simpler route... I plugged them in.

The Bag End S15 series cab ( for electric bass) are a marvel IMO, both coax and full range. Adding the even more tiny Nahas 1-12 is such a perfect match it blew me away the first time I tried it and it's still blowing people away to this day. It doesn't "look" like it could possibly sound and loud as it does. And the tone? Geez.

I play 4,5 and 6 string basses. Sometimes with PA support, just as often not. My little big rig has held it's own in a bass/bass/ drums trio with a two 410 stack volumewise and again the tone... no contest. It really is my big rig.

They complement each other very well. The 15 does some things the 12 doesn't and vice versa. The 15 (no horn) has excellent lows and mids and pretty good highs, which the 12 extends. The 12 (with horn) has excellent highs, mids and pretty good lows... which is where the 15 comes in. IME you don't get that kind of overlap with most 1-10s or smaller.

Joey Nahas made the tiltback 1-12 I use and since I found the Grail he made a 1-15 similar to the Bag End S-15D but 3" deeper. Still rear ported, it's loaded with a cast frame Emminence driver that goes both lower and higher than the BE did. A great match for the Nahas 1-12. The bonus is that this 1-15/1-12 rig including amp weighs ~100 lb.

I've been messing around with a pair of single 12 coaxes (EA CX112, homemade w/Selenium driver) over a pair of EV 1 X 15 cabs, powered 800 watts/ per side. The overlap is where the magic is, in my experience. My EVs are different models;nothing matches, sounds amazing.